The present invention relates to an ultrasonic moving object detection system and more particularly, to a method for detecting the presence of moving objects through utilization of ultrasonic waves or energy with employment, for example, of a micro-processor, and an apparatus employed for effecting said method in which the detecting period is automatically set, especially when the presence of vehicles and the like is to be detected on the basis of reflection of ultrasonic energy directed to such vehicles to be detected.
In general, in order to detect the presence of moving objects such as vehicles or the like, for example, in a traffic survey, ultrasonic waves are directed toward a surface of the road for detection of the presence of vehicles on the basis of the time difference in arrival of reflected waves from the road surface and from the vehicles in the presence of such vehicles. More specifically, in FIGS. 1(a) to 2(d) explanatory of a known arrangement for moving object detection with the employment of an ultrasonic detector, a detecting unit UL selectively transmitting and receiving ultrasonic energy is provided at a height Ha from a road surface R for transmitting the ultrasonic energy from said detecting unit. In the absence of a vehicle V as shown in FIG. 1(a), the ultrasonic detector unit UL receives or detects reflection of waves from the road surface R, and if the time between the time point of ultrasonic wave transmission and that of reception of the reflected waves is designated as .tau.a, such time .tau.a is represented by the following equation (1), EQU .tau.a=2Ha/C (1)
wherein C is the second velocity which is equal to 331+0.6T(m/s) and T is the ambient temperature (.degree. C). Meanwhile, in the presence of a vehicle V as shown in FIG. 1(b), the detector unit UL receives waves reflected from such a vehicle, and if the height of the vehicle V is denoted by Hc, and the height from the vehicle to the detector unit UL by Hb (Hb=Ha-Hc), with the time between the time point of the ultrasonic wave transmission to the reception of the reflected waves being designated as .tau.b, such time .tau.b being represented by the following equation (2), EQU .tau.b=2Hb/C=2(Ha-Hc)/C (2)
Therefore, the presence of vehicles may be detected by detecting whether the time interval between the transmission of the ultrasonic waves from the detector unit UL and the reception of the reflected waves by said detector unit UL is equal to the time .tau.a or .tau.b (.tau.a&gt;.tau.b). As is seen from FIGS. 2(a) to 2(d) showing time charts for vehicle detection with the use of ultrasonic energy, time .tau.o for one period of transmission is selected to be larger than the time .tau.a between the time point of the ultrasonic wave transmission and the time point of the reception of the reflected waves from the road surface R, while the reflected wave detecting time .tau.b in the presence of the vehicle V is smaller than the time .tau.a mentioned above. For accurate detection of the wave reflected from the particular vehicle, however, it is necessary to properly define a gate period .tau.v during which the vehicle can be detected. Such vehicle detection gate period .tau.v is later than time .tau.l which is the sum of the time of the transmission and of the reverberation developed by the detector UL due to inertia immediately after said transmission and is selected to be slightly earlier (by the time .tau.h for preventing wrong detection, for example, of a raised road surface due to accumulation of snow as the presence of a vehicle) than the time point for the detection of the waves reflected from the road surface R. In the conventional arrangement as described above, however, proper adjustments are required in the installation of the detector unit UL, since the vehicle detection gate period .tau.v differs depending on the installing positions of such detector unit UL although the time periods .tau.l and .tau.h are of fixed nature, and such adjustments are conventionally carried out by skilled technicians at the site of installation with the use of an oscilloscope and the like through complicated procedures. As is seen from the foregoing description, in the known detecting apparatuses utilizing ultrasonic energy, it has been extremely difficult to properly set the vehicle detection gate period .tau.v, and even if once properly adjusted, such vehicle detection gate period .tau.v must be corrected frequently due to alteration of propagation velocity of the ultrasonic waves by variations of sound velocity through fluctuations of external factors such as ambient temperatures.
In other words, in the detection of presence of vehicles by utilizing ultrasonic energy, it is commonly arranged that, in order to eliminate reverberation of the transmitted ultrasonic energy and influence of the reflected ultrasonic energy from a surface of a road, a wave-reception gate or detection gate of an ultrasonic transmitter-receiver is opened after an elapse of a predetermined period of time subsequent to transmission of ultrasonic energy, while such a detection gate is closed after an elapse of another predetermined period of time, so that when the reflected energy is received while the detection gate is kept open, the particular reflected energy is regarded as that coming from a vehicle for the detection of presence of such vehicles.
It should be noted here, however, that the velocity of propagation of ultrasonic waves varies depending on ambient temperatures and the like as mentioned earlier. Therefore, if the duration for opening such detection gate is fixed on the basis of the propagation velocity of ultrasonic energy at a given standard ambient temperature as in the known arrangement, there is a possibility that the ultrasonic energy reflected from the surfaces of roads may be received before the detection gate has been closed, for example, due to increase of the propagation velocity of the ultrasonic energy through variations of ambient temperatures, while on the contrary, if the propagation velocity is reduced, the ultrasonic energy reflected from a vehicle may reach the transmitter-receiver after the detection gate has been closed.
Therefore, an accurate detection of traffic would be made feasible if it were possible to detect the time of flight of reflected energy from the road surface to the transmitter-receiver in response to the variation in propagation velocity of ultrasonic energy and discriminate any reflected energy arriving with a time shorter than said flight time by a preset time span as energy from the traffic. For the above purpose, however, it is necessary to keep constant tabs on the flight times of reflected energy from the road surface to the ultrasonic transmitter-receiver.
In view of the foregoing problem, it is intended in the present invention to provide a method for automatically setting a standard plane which is such that regardless of any change in the flight time of reflected energy from the road surface to the transmitter-receiver, such flight times are automatically detected and set by means of a micro-processor, each flight time of reflected energy is compared with the set time and, when any particular flight time is smaller than said set time by more than a preset value, it is discriminated as energy reflected back from the traffic.
The term "standard plane" is used herein to mean the time after transmission of ultrasonic energy till the arrival of the corresponding reflected energy at an ultrasonic transmitter-receiver. Thus, the distance between the transmitter-receiver and the road surface is actually constant and, hence, the flight time of reflected energy from the road surface to the transmitter-receiver should be constant if the propagation velocity of ultrasonic energy be constant. However, a variation in flight time of reflected energy due to variations in the propagation velocity of ultrasonice energy may be regarded as a change in distance between road surface and transmitter-receiver. It may also happen that the surface of reflection is different from the actual road surface. If a variation in propagation velocity of ultrasonic energy be thus assumed to be a change in the relative distance between road surface and transmitter-receiver and the road surface be regarded as a standard plane, any reflected energy arriving within a time span shorter than the flight time of reflected energy from said standard plane by more than a preset time value may be discriminated as reflected energy from the traffic. The term `standard plane` as used hereinbelow has the above meaning, namely the time after transmission of ultrasonic energy till the arrival of corresponding reflected energy at the transmitter-receiver.